New Mexico Mineral Symposium — Abstracts

Paleozoic carbonates (minimum age 449 rn.y. ± 16 mey.) intrude the Precambrian rocks exposed in the eastern Lemitar Mountains, Socorro County. Carbonatites are unique carbonate-rich rocks of apparent magmatic origin and are characterized by a distinct but variable mineralogya chemistry, and associated alteration. The Lemitar carbonatites occur as dikes, stockworks, and veins and display textures, mineralogy, chemistry and wail-rock alteration typical of carbonatite complexes. They contain greater than 50% carbonate minerals and varying amounts of apatite, magnetite, pyroxenee, and other accessory minerals. Despite variations in texture of the Lemitar carbonatites, they can be grouped on the basis of mineralogy and mode of emplacement as: 1) silicocarbonatite dikes; 2) sovite (greater than 90% calcite), ravhaugite (greater than 90% dolomite), and carbonatite veins; 3) ankerite-dolomite carbonatite dikes; and 4) stockwork carbonatites.

Barite-fluorite-sulfide veins occur in the vicinity of the carbonatite dikes and locally occur with some carbonatites. The primary mineralization of the veins is barite, fluorite, galena, sphalerite, quartz, and small amounts of chalcopyrite, pyrite, and calcite. Many of the veins contain silver. Subsequent alteration has produced wulfenite, hemimorphite, cerussite,- hematite, and a small amount of malachite, chrysocolla, and anglesite. Barite-fluorite-galena mineralization is common in central New Mexico and is considered to be Tertiary in age. This mineralization is probably formed by the expulsion of basinal brines along fractures related to the Rio Grande Rift. However, it is possible, that some of the barite mineralization in the Lemitar Mountains is related to the carbonatites and is of Paleozoic age.

The presence of carbonatites in the Lemitar Mountains may also have tectonic significance. Carbonatites are commonly associated with continental rifts or lineaments and the carbonatites in Socorro County support current theories of rifting occurring as early as Precambrian or Paleozoic. Other carbonatites and alkalic rocks in New Mexico and Colorado are similar in composition and age to the Lemitar carbonatites, further supporting Precambrian or early Paleozoic rifting.